This invention relates to a method of control of cooking apparatus, and in particular to control over the heat transfer rate to food products being cooked in a particular cooking apparatus such as frying apparatus. Cooking apparatus for implementing the method is also disclosed.
Cooking apparatus cooks food items by exposing such to a heated liquid cooking medium such as an oil, gas or molten solid fat. The liquid may be oleaginous or aqueous depending on the nature of the cooking process, whether boiling or frying. Gaseous media may include steam or heated air or other gases. Conventionally, the cooking process -achieved by transfer of heat to the food material - is controlled by controlling two variables, the temperature of the cooking medium; and the time for which food items are exposed to the cooking medium.
A number of problems may arise with this form of control in practice. For example, while temperature may be controlled it has generally been controlled in a somewhat imprecise way, the temperature being that guessed by the operator to be the optimum for the cooking process on the basis of minimal temperature data. A similar imprecision applies to the cooking duration. The duration may be approximately correct but not optimum. Then, a still further level of complexity and lack of precision may result where there is no single cooking temperature that is applicable throughout the entire cooking process but rather the temperature should be a function of time, that is, changing throughout the cooking process. By way of example might be mentioned the cooking of a meat item where it may be desired to seal the outside of the item at high temperature at the commencement of the cooking process, the temperature then being varied downward to a value which is then maintained for the remainder of the cooking process.
Further variability in cooking temperature may result from temperature variations caused, for example, by loading of frozen food items into the cooking chamber or cooking zone during the cooking process. The loading operation will cause temperature drop on contact of frozen items with heated cooking medium. This problem that requires to be addressed.
A vat frying application, as commonly used in the quickservice food industry, is one employing the principle of deep frying of food items by immersing them in a body of heated cooking medium. Deep frying is a process of significant concern to the Applicant. In that case, a difficulty arises that is largely accepted in conventional practice, that is control over the temperature at one specific location within the body of cooking medium. This is a compromised system in which local overheating and underheating may take place, not only compromising the efficiency and quality of the cooking process and the food items that result from it but also leading to problems with the operation of the equipment itself. Even if temperature control is attempted, thermal inertia of the cooking medium filled vat results in poor control response and unbalanced temperature profile.
If there is local overheating of cooking medium, food items may be overcooked or may be non-uniformly cooked. If temperature is high local to a heating element, food particles and crumbs in this location may carbonise causing a cleaning and cooking medium quality problem. The cooking medium may degrade, product texture may be adversely affected or the oil uptake into the product may be increased. The latter situation may be undesirable where oily food product quality is desired to be avoided. Overheating may be a particular problem where carbonised food particles deposit in the pipes conveying the cooking medium within a given cooking apparatus.
In vat fryers, complete dismantling, cleaning or replacement of the pipes might be required every three to five years or even less depending on the capacity and usage of the cooking apparatus. In any event, any deposition of xe2x80x9cscalexe2x80x9d will interfere with the efficiency of heating of the cooking medium by a heat exchanger and may be compensated for by techniques that may actually exacerbate the overheating problem, for example heating of the heat exchange element or heating medium to higher temperatures where overheating becomes even more likely.
Previous practice has aimed at addressing such problems by mechanical design measures such as scrapers and augers for removal of crumb deposits. Oil removal devices may also be required. For example, vats may be equipped with cross flow arrangements aimed at causing mixing of the cooking medium and a more uniform heating. Cold zones might be provided in regions where crumbs are likely to deposit such that the temperature is maintained below the carbonisation temperature. Steam purging might also be employed in an effort to prevent carbonisation but this technique is potentially dangerous. In addition, filtration systems such as drum filters, belt filters (with paper or cloth belts) or other complex filtration systems may be installed. Drum and belt filters which expose oil to air are undesirable.
It is the object of achieving better control over the cooking process by reducing the above problems with the minimum resort to complex mechanical arrangements that forms the basis of this invention.
With this object in view, the present invention provides a method of at least partially cooking food items by exposing them to a heated liquid cooking medium supplied by cooking medium delivery means to a cooking zone of a cooker including a control unit which, during a cooking process;
(a) controls the temperature of the cooking medium delivered to the cooking zone to a pre-determined setpoint specific for the food items by controlling heat output from a heat exchanger for heating cooking medium throughout the cooking process;
(b) controls the rate of convective heat transfer to the food items by directly controlling a sensed condition of cooking medium delivered to the cooking zone, other than temperature which is controlled in step (a), related to the rate of convective heat transfer; and, optionally,
(c) controls the rate of convective heat transfer to the food items by controlling a sensed condition of the cooking zone related to the rate of convective heat transfer.
Temperature control may be achieved throughout the cooking process by appropriately heating the cooking medium to a pre-determined setpoint for the cooking process using suitable heat exchangers which are operated bearing in mind the particular nature of the cooking medium being heated. For example, in the case of frying, where the cooking medium is an oil or molten fat, heating is desirably conducted in the absence of air using, for example, the flow heater arrangements as described in our Australian Patent No. 666944; and co-pending International Patent Application No. PCT/AU98/00552, filed 16th Jul., 1998, the contents of which are hereby incorporated herein by reference. Such heat exchangers may include a cooking medium cooling sub-system. Temperature may be varied as desired throughout the cooking process by suitable control over heat exchange element output.
The control over the rate of transfer of heat from cooking medium to food item may be achieved in a number of different ways. In establishing this control, the applicant has recognised that the cooking process commonly proceeds by a convective heat transfer mechanism rather than a conductive heat transfer mechanism. The thermal conductivity of cooking media such as oils and fats is quite low. Indeed oils and fats are insulators which only poorly conduct heat to food items being cooked.
Mechanisms of convective heat transfer rate control predicated on this understanding, must advantageously take into account the cooking process, the nature of food items to be cooked, such mechanisms may include the induction of turbulence in the cooking, medium especially local to a food item being cooked or, particularly desirably, control over the pressure or rate of flow of cooking medium delivered to the cooking zone or speed of flow of cooking medium past the food item. Such control may particularly advantageously be achieved independently of the control over cooking medium temperature and cooking duration, the latter of which is also controlled.
More specifically, said sensed condition of cooking medium delivered to the cooking zone is one or more of the following: cooking medium delivery pressure, cooking medium flowrate, cooking medium density, cooking medium viscosity and cooking medium turbulence; and, where step (c) is employed, one or more of the following: sprayer type, nozzle type, hydraulic restriction type, turbulence in the cooking zone, cooking zone temperature, cooking duration and basket shaking frequency.
Such control may be applied throughout the cooking process in the cases of various different cooking apparatus types. Indicative of the types of cooking apparatus that may be controlled in accordance with the method are spray cooking apparatus, vat type cooking apparatus and pressure fryers. Blanching apparatus may also be controlled in this way. The method may be applied to other kinds of cooking apparatus operating on the basis of cooking processes in which a convective heat transfer mechanism is of importance. Most cooking processes would fall into this category.
In the case where the cooking apparatus is a spray cooking apparatus, for example a spray fryer, the cooking medium flow rate through the sprayer nozzles into the cooking chamber may be controllably varied as may the cooking medium delivery pressure. Such sprayer nozzles may be of different type in terms of flow distribution and operating pressure range.
In the case where the cooking apparatus is a vat cooking apparatus, for example a vat fryer, turbulence or speed of flow of cooking medium past the surfaces of the food item while being cooked and energy transfer rate may be controllably varied. Generally, increasing speed or flow results in higher convective heat transfer. In this case, the sprayer nozzles may be replaced by hydraulic, such as flow or pressure, restrictions within the cooking medium delivery means to the vat. Such restrictions, like sprayers and nozzles, may have different characteristics such as pressure head and so on. The diaphragms, for example, may be employed in vat arrangements including crossflow arrangements for fine tuning of cooking medium flow which may be varied through the vat during the cooking process, to ensure substantially uniform temperature profile therein.
Cooking apparatus may be of multiple zone nature where the apparatus is configured to allow different cooking methodologies for food items progressing through the cooking zones through the generic cooking mechanism, for example spray or vat frying, may be the same for each cooking zone. This takes account of the fact that cooking processes often change in nature during the optimum cooking of some food items. Energy and temperature profile across the constituent cooking zones may be controlled to achieve this object. A common example applicable to meat is initial searing followed by slower cooking. Another food item type which undergoes different regimes during optimum cooking is potato crisps.
Advantageously, energy absorbed by the food items during the cooking process may be measured and used as a further basis for controlling the operation of cooking apparatus. In particular, such measurement may advantageously be used as a basis for controlling the rate of transfer of heat to the food items during the cooking process.
The measurement of energy absorbed by food items may be determined on the basis of the temperature differential between cooking medium temperature entering the or each cooking zone and cooking medium temperature leaving the or each cooking zone. This temperature differential may be multiplied by the mass flowrate of cooking medium and by the specific heat of the cooking medium. Allowance may also be made for heat losses from the cooking zone.
If the temperature differential falls outside predetermined limits, then corrective action may be taken to bring the temperature differential and the energy absorbed by the food items in the or each cooking zone within the desired limits.
Corrective action may be taken firstly by altering the flowrate of cooking medium, thus to decrease the error between the desired energy absorption of the food items and that actually achieved. An alarm condition may also be indicated at the same time.
Less preferably, as the cooking medium is heated in a heat exchanger to the desired temperature, temperature of the input cooking medium to the cooking zone may be varied in order to decrease the error between the desired energy absorption of the food items and that actually achieved. This is a more compromised control response because the increase of temperature of the cooking medium may have disadvantageous effects on the cooking medium quality.
The cooking duration, a sensed condition of the cooking zone like turbulence, may also be varied by, for example, controlling the speed of conveyor means or food product delivery or transfer means as a possible control response. The control responses are not exclusive. In addition, those described may be utilised in any combination.
The control strategy may be used to check for blockage of nozzles and other devices for delivery of liquid cooking medium to the cooking zone. When nozzles block, the temperature differential varies from normal conditions. The temperature differential could be measured between three temperature conditions of the cooking media: temperature of the liquid delivered to the cooking zone; temperature of the liquid in the cooking zone; and temperature of liquid leaving the cooking zone. As a result energy absorbed by food items may fall and food quality itself may vary since there no longer occurs a desired spraying or distribution of cooking medium into the cooking zone(s) of a cooker.
The control strategy is advantageously implemented by an electronic control unit used for the overall control over the operations of the cooking apparatus. It is most advantageous and desirable that the method be fully automated to achieve the best possible efficiencies of cooking. In particular, the control unit implements desired temperature, cooking medium flow and heat transfer rate control.
The control unit may be programmed with target or setpoint cooking medium temperature differentials specific to a given cooking process. The temperature differential setpoints may be programmed as a function of time, cooking medium flow rate, food item nature, food item throughput and/or other variables.
In a second aspect of the invention, relating to a most preferred cooking apparatus, there is provided a cooking apparatus including a cooking zone; cooking medium delivery means for delivering cooking medium to the cooking zone; cooking medium removal means for removing cooking medium from the cooking zone; and cooking medium conditioning means for conditioning removed and fresh cooking medium for delivery to the cooking zone wherein the means includes in sequence, pump means; filtration means and heat exchange means to which at least a portion of the cooking medium being conditioned successively passes.
The pump means, filtration means and heat exchange means, which may take the form of modules containing any desired number of pump, filtration and heat exchange units may be arranged in series or parallel as desired.
The pump means is particularly advantageously a variable speed centrifugal pump. The filtration after pumping, pressure filtration, contrasts with previous practice which required on the suction side of the pump a large filter area comprising a coarse primary filtration prior to pumping to protect the pump means and a fine secondary filtration following pumping and prior to heat exchange. Single stage filtration after pumping may be employed in accordance with the present invention reducing the filter area and allowing achievement of a more compact system.
The speed of the pump may be controlled having regard to pressure sensed in the cooking medium delivery means to the cooking zone, that is prior to the flow, pressure or hydraulic restrictors, sprayers or inlet nozzles depending upon the nature of the cooking apparatus. The centrifugal pump is particularly suited for this application because impeller speed can be varied to maintain the cooking medium flowrate as filter pressure rises without deterioration in the cooking medium quality, especially where the cooking medium is an oxidisable or otherwise degradable medium such as a fat or oil. In fact, the Applicant has measured no substantial deterioration in oil quality at pressures up to 5 bar. The cooking medium flowrate may be maintained substantially constant or varied for any or all cooking zones to achieve the object of controlled heat transfer rate to food items. Temperature of the cooking medium is likewise controlled, for example, by controlling the heat exchange element output or heating medium of the heat exchanger to achieve the optimum temperature or temperature range for the cooking medium for the particular application throughout the cooking process. Control over cooking medium flowrate and speed through the heat exchanger may also be conducted to achieve the desired cooking medium temperature.
It will be understood that the variables of flowrate, heat exchange element output and cooking medium pressure and flowrate may be varied to achieve varying heat transfer rate throughout the cooking process if this is desirable having regard to the nature of the cooking process and/or the nature of the food items being cooked.
The measured pressure may typically be a function of the degree of clogging of the filtration means and, even irrespective of pump characteristics, the filtration means may be operated to allow cleaning routines such as flushing or replacement if pressure sensed in the cooking medium delivery means falls below a predetermined value. These cleaning routines or replacement might also be indicated where pump impeller speed, increased to compensate for falling pressure in the delivery means, exceeds ac predetermined value. Other techniques for determining this point might be employed, for example optical or ultrasonic testing of clogging of the filter and so on.
The cooking apparatus may include means for measuring the energy absorbed by the food items during cooking. Thus, for example, temperature sensing means for sensing cooking medium temperature may be arranged such that temperature differential between delivered and removed cooking medium may be determined and used in calculation of absorbed energy as above described.
The method and apparatus of the above aspects of the invention may conveniently be supervised by a control unit for the cooking apparatus which implements desired temperature, flow and heat transfer rate control; and/or filtration means operation. The control may be made with reference to the nature of the food items being cooked, the cooking medium, the desires of the consumer for particular food products at various locations in which cooking apparatus operated in accordance with the method of the invention is located. The control unit may be electronic and used for calculating temperature differential and maintaining it within desired limits. The method may also be used with cooking apparatus other than that of the second aspect of the invention.